The United States is to intensify its military build up in the Persian Gulf by deploying miniature underwater drones to hunt for Iranian mines.

The four-foot-long, remotely guided "SeaFox" submersibles are expected to form a key element of US military strategy in the region amid mounting tensions over Iran's nuclear programme.

Tehran has repeatedly threatened to respond to Western sanctions by mounting a blockade of the Strait of Hormuz, the world's most important energy waterway.

With the failure of diplomatic efforts to persuade Iran to curb its nuclear ambitions, the United States has gradually begun to reinforce its military assets in the Gulf.

Recent weeks have seen a significant increase of American's air, sea and ground assets. An unspecified number of F-22 stealth fighters and a "mother-ship" that could serve as a floating barracks for up to 900 soldiers were moved into the region earlier this month.

The SeaFox drones will significantly bolster the US military's countermine capability, which was also boosted this month with an announcement that four minesweepers would sail into the Gulf, doubling the number to eight.

The deployment of the submersibles reflects the concern of American military planners that mines would pose the greatest threat to commercial maritime traffic were Iran to seal the Strait of Hormuz, a choke point through which a fifth of the world's tanker-borne oil is ferried.

History suggests that even American naval vessels could be vulnerable.

During the Tanker War of the 1980s, Iranian mines struck several commercial vessels with one badly damaging the USS Samuel Roberts, a guided missile frigate.

The remotely guided, unmanned submersibles, which weigh only 88lb, seek out their targets with a homing sonar. When contact is made, the drone explodes, obliterating itself as well as the mine.

The first drones, which are designed by a German company once owned by the British defence giant BAE Systems, have already arrived in the region, the Los Angeles Times reported.

The SeaFox, which is controlled by a 3,000-foot fibre-optic cable is also used by the Royal Navy.

Iran's minelaying capability has been enhanced in recent years after it built 19 mini-submarines that are difficult to track. Even so, many analysts question how long Iran would be able to sustain a blockade that would also damage its own economy by preventing Iranian oil tankers leaving the Gulf.

America's military build up is seen in part as an attempt to calm Israel, which has threatened to launch unilateral military action against Iran.

But with Western intelligence assessments concluding that Iran is potentially just a year away from building a nuclear weapon, observers say it is also a sign of the growing likelihood of a military confrontation to resolve the crisis.